Technical Field
The technology relates to methods and structures for performing etching of sacrificial layers of III-nitride material. The etching techniques may be used for micro- and nano-fabrication of integrated devices, such as vertical cavity surface emitting lasers (VCSELs) based on III-nitride semiconductor material. The VCSELs may include air/semiconductor, distributed Bragg reflector (DBR) structures formed adjacent to the VCSEL using lateral, electrochemical etching techniques.
Discussion of the Related Art
The etching of semiconductor materials is an important technique that is used in microfabrication processes. Various kinds of etching recipes have been developed for many materials used in semiconductor manufacturing. For example, Si and certain oxides may be routinely etched using dry (e.g., reactive-ion etching) or wet chemical etching techniques that yield desired etch rates and etch morphologies. III-nitride materials have recently emerged as attractive materials for semiconductor manufacturing, however these materials can be chemically inert to standard wet etchants.
Some attractive applications for III-nitride materials include micro-photonic devices, such as LEDs and lasers. Some existing methods of making III-nitride VCSELs involve forming structures that comprise alternating layers of AlGaN/GaN or AlInN/GaN. However, these structures are difficult to produce and do not exhibit high refractive index contrast between the alternating layers. Accordingly, to obtain a suitable reflectivity for a laser cavity, the number of layers must be increased (e.g., to about 40) making the overall cavity thick. Additionally, it is difficult to match the cavity mode with an active layer emission wavelength. Another approach to making a VCSEL with a DBR structure is to form dielectric DBRs at two ends of the cavity using layer lift-off techniques. However, this approach is complex to implement and suffers from low yield during manufacture.
As InGaN light emitting diodes (LEDs) gradually approach technological maturity in performance for blue/green emissions, microcavity based LEDs (resonant-cavity LED, RCLED) and laser diodes (vertical-cavity surfaceemitting laser, VCSEL) become appealing alternatives that may offer advantages in enhancing radiative recombination rates, improving beam directionality, and possibly reducing the cost in manufacturing due to their planar configuration.
The encasing of the optical active region into a cavity of a few wavelengths may be done using distributed Bragg reflectors (DBRs) with minimum absorption and high reflectivity. For nitride-based emitters, top p-side DBRs may be prepared by depositing dielectric, quarter-wavelength stacks as a last step of device fabrication. The bottom (n-side) DBRs may be implemented using either dielectric stacks or epitaxially grown (Ga,Al,In)N/GaN periodic heterostructures. However, challenges still exist in both approaches toward ultimate manufacturability. The dielectric approach can require complicated thin-film lift-off and wafer bonding in order to expose the n-type GaN. For epitaxial DBRs, 20 to 40 pairs of heterostructures are needed for good peak reflectance (above 95%) due to the small contrast of refractive indices. Such a thick DBR structure causes a narrow stopband (<50 nm), creates issues in stress management on the device, and reduces the benefit of the Purcell effect.
Semiconductor/air structures have been pursued using a photo-assisted electrochemical (PEC) etch where mirrority-holes are photo-generated and confined in narrower-bandgap sacrificial layers to facilitate selective etching. Also, selective chemical etching has been identified for the AlInN/GaN system. The membrane structures prepared by the two techniques generally suffer from etched surface roughness that contributes to scattering losses in the optical devices. The maximum reflectance in the blue/green range from GaN/air DBRs, prepared by either PEC or selective wet etching, has not exceeded 75%.